Interference suppression network



Sept. 19, 1939. J. w. MILNOR r Al. 2,173,722

ITERFERENCE SUPPRESSION NETWORK Filed Jan. 28, 1936 2 Sheets-Sheet 1 L ine .82 .84 .66 .88 .90 .92 .94 .96 .96 v1.0 1.021.04 1.06 1.06 1.l0-l.12 1.15 1.16 1.18 f7.6 a ,7 C 7Z//7eo/ Fregueracy mmwv" Z 9 J. W Mawr flwmmm Sept. 19, 1939. J. w. MILNoR E-r AL INTERFERENCE SUPPRESSION NETWORK Filed Jan. 28, 1936 2 sheets-sheet 2' J: W zlno:Q P. if echberger 7V.' AD. Cannonv Patented Sept. 19, 1939 UNITED STATES PATENT OFFICE ger, Metuchen,

Bergenield, and William D. Cannon, N. J., assignors to The Western Union Telegraph Company, New York, N. Y., a corporation of New York Application January 28, 1936, Serial No. 61,234

16 Claims.

This invention relates to an interference suppression network and in particular to a bridge type of interference suppression netwo-rk adapted especially for application to duplex telegraph sets operating on grounded lines.

Broadly, the object of the invention is to provide means for excluding from the receiving relay circuit of a telegraph system disturbing currents induced into the line conductor by neighboring power or other interfering circuits. For accomplishing this purpose, specific objects of the invention are to provide a balanced bridge network composed of four tuned circuits, and in which one or both pairs of resonant opposite arms are mutually coupled, to provide a Wheatstone bridge network in which all the tuned arms have impedances equal at resonance and of the same order as the impedances of the line and terminal apparatus, to provide a balanced bridge tuned network in which a single tuned circuit is coupled to a plurality of arms so as to tune each of said arms to the disturbing frequency, to provide a balanced bridge network in which a single condenser serves to tune two arms of the bridge so as to provide circuits anti-resonant to the disturbing frequency.

The invention is applicable to a telegraph system comprising a plurality of duplex telegraph sets, and in this embodiment the object of the invention is to provide a single network ofthe type described which will be applicable to a plurality of lines, the network having the form of a plurality of Wheatstone bridges having certain elements in common.

Networks of the type of this invention in which all of the arms of the balanced bridge are formed of tuned circuits have the advantage that the efciency of the network is considerably improved over that obtainable with networks of the types disclosed in Patents 1,773,371 and 1,766,919, in which combinations of resistances and tuned circuits are used. Furthermore, the use of common cores as disclosed in this invention avoid excessive bulk and unnecessary expense while still maintaining a high degree of eiiiciency in the elimination of power interference and preserving the balance of the telegraph sets.

The interference suppression network according to the invention has the characteristics of a Wheatstone bridge which is balanced at the frequency of the disturbing current, and unbalanced at signal frequencies. The receiver circuit being connected across the diagonal of the balanced bridge is in conjugate relation to the line conductor circuit for currents of the frequency for which the bridge is balanced, and is, therefore, unaffected by currents of disturbing fre` quency incoming from the line. At the same time the lack of balance at signal frequencies makes the relay circuit receptive to incoming currents of signal frequencies, the effect of the network on these currents being only to introduce a negligible loss into the circuit.

By proper design of the network it has been possible to produce a direct current telegraph system operating at a dot frequency higher than an interfering frequency having a current magnitude of the same order or higher.

Referring to the drawings:

Figure 1 represents a duplex telegraph set with an interference suppression network designed in accordance with this invention;

Figure 2 shows a group of curves indicating the eiect of the factor Q of the network in determining the overall performance of the device;

Figure 3 represents a telegraph system cornprising three lines with a single interference suppression network connected to all three lines;

Figure 4 illustrates the circuit of Figure 1 arranged in a different manner for the purpose of explaining its operation;

Figure 5 is a simplified diagram illustrating the principles of the same circuit shown in the conventional form of a Wheatstone bridge.

In Figure 1 there is shown a duplex set of input impedance Z connected to a line of like impedance, with an interference suppressing network interposed between them. LC represents the line conductor over which telegraphic messages are to be received and transmitted. The line conductor is subject to disturbing current induced therein by neighboring power circuits or the like. The line is balanced by artificial line AL so as to render the transmitter TR and the receiver conjugate with respect to transmitted currents. The receiver is diagrammatically illustrated by windings W1 and W2 and an associated armatura'the transmitter TR being connected betweenground Gand the junction point 5 of the receiver relay windings W1, W2. Terminals I, 2, 3 and. 4 are terminals of the interference suppression network. The line LC and artificial line AL are connected, respectively, to terminals l and 2. The ends of windings W1, W2 remote from junction point 5 are connected to terminals 4 and 3.

The interference suppression network has four tuned arms; these arms will be identified hereinafter by the terminals of the network to which they Yare connected. Thus, the series resonant arms are arms 3 and 2 4, while the antiresonant arms are arms I fl and 2 3. In Fig. 1 each arm, 1 3 and 2 4 comprises the inductance of a coil l2 and the capacity of a tuning condenser. Coil I2 is composed of two windings ila and Hb mounted on a common core H. Condenser I3 in series with winding Ha. of coil l2 forms the series resonant arm I 3, while condenser Il! in series with winding Hb of coil I2 forms the opposite, series-resonant arm 2 l`. The other pair of arms is composed of anti-resonant circuits including coil I6. Coil I6 is made up of three windings I5a, lband l5c on a common core l5, the third winding l5c of which is closed by condenser l1 to form a resonant circuit. Windings I5a and I5b of coil I6 are connected in arms 4 and 2 3 respectively, and by reason of the coupling provided by the common core l5 between the tuned circuit of winding Hic and the two arms, both arms are adjusted to anti-resonance for the disturbing frequency which is to be suppressed. The arms of the 'tuned network are so adjusted that they all have impedances equal at resonance and approximately equal to the impedances of the line and terminal apparatus. That is, if Z1, Z2, Z3 and Z4 represent the impedances of the four resonant arms of the network, as shown in Fig. 1, these impedances at resonance all have the same value and are approximately equal to the input impedance Z.

As is well known, this proportion of the elements of a Wheatstone bridge provides maximum sensitivity. .At frequencies other than resonance the bridge is unbalanced, the degree of unbalance and consequent loss introduced being dependent upon the rigidity with which the values of the elements adhere to the requirements as set forth above.

The operation of the circuit arrangement illustrated in Figs. 1 and 4 is as follows:

The two tuned arms and the two anti-resonance arms are adjusted to resonance with the disturbing currents, which are usually alternating currents of or 60 cycles frequency induced by neighboring power lines. Under theforegoing conditions, the bridge will be balanced with respect to disturbing currents and the receiving relay will not be operated thereby. However, for incoming telegraphic signals the bridge will not be balanced and signal currents will flow through the windings W1 and W2 of the relay and effect its operation. Upon transmitting signals from transmitter TR, the transmitted current ows through windings W1 and W2 in opposite directions and therefore the receiving relay will not be operated. Artificial line AL is adjusted in impedance to produce a balanced relation between transmitter TR and the receiving relay. In other words, transmitter TR is in conjugate relation with the receiving relay.

For the purpose of obtaining the desired values of resonant impedance the quality of the coils becomes important. This property can be represented by a factor Q equal to 21rfL Rao ance of the device. The effect of the circuit Q is indicated in the family of curves of Fig. 2.

In Fig. 2 the abscissae represent frequency, and the ordinates represent insertion loss due to the network. As shown in the figure, maximum loss occurs at frequency 1.0 which represents the interfering frequency. The curves represent the loss with frequency for different values of Q, the particular values of Q for'which the curves are shown being represented as 10, 15, 20, 25 and 30. It can be seen that with coils of relatively high quality a very high degree of suppression of an interfering frequency is achieved.

Coils of high quality (large Q) at power frequencies are rather large in size. Furthermore, to construct a tuned network having separate inductances in the tuned arms would require inconveniently large values of condensers in the anti-resonant circuits. By combining the coils of arms 2 4 and 1 3 into a single coil I2, and the coils of arms 1 4 and 2 3 into a single coil I6 as shown in Fig. 1, the number of coils is halved and the large condensers of the anti-resonant circuits can be replaced by a single condenser I'i of the order of one microfarad. The total inductance of the two matched windings in series of the coil l5 should be this Q being the Q for the circuit of the third winding. The inductance of this winding can be made such as to tune with a convenient value of capacity, and having also a maximum Q at the frequency to be rejected. The inductance of the two windings of coil i2 in series should be Q here being applicable to the entire circuit.

The network described above is particularly effective for the suppression of power interference of 25 and 60 cycles. Satisfactory operation of duplex telegraph sets at speeds which closely approach the interfering frequency has been achieved. In fact it has been found possible to operate wires at speeds higher than the interfering frequency at only slightly reduced efiiciency.

A still further economy in the use of this type of interference eliminating network can be achieved by adapting a single network to a plurality of wires. One method for doing this is shown in Figure 3 in which three lines are connected to a single network. The series coil contains separate windings for each line and for the associated artificial line connections, but in the diagonal coil a single winding serves for each of the three lines. The three lines are then isolated by means of the condensers.

Thus, in Fig. 3, three line conductors Illa, Illb and ille are connected with their respective artificial lines through a network which forms a plurality of Wheatstone bridges one for each duplex telegraph set, the three bridges having the coils 20 and 4E) in common. Each line has its own transmitter T1, T2 or T3 and its own receiver R1, R2 or R3, represented diagrammatically by a pair of windings and an associated armature, in the same manner as shown in Fig. 1. The line is balanced in the usual manner so that signal current from a transmitter T1, T2 or T3 has no effect on the associated receiver R1, R2 or R3. Between each line and artificial line is connected a Wheatstone bridge network designed according to this invention, the terminals of the network for line a and its artificial line connected to conductor |9a being la, 2a, 3a and 4a. Similarly, the network terminals for line conductor Ib are lb, 2b, 3b and 4b and the network terminals for line conductor iic are ic, 2c, 3c and 4c.

Coil 20 has two windings 2|, 22 mounted on a common core 23. The ends of winding 2| are connected to conductor Ilia and its associated articial line conductor ida through the condensers 24 and 25, to form the series resonant arm 2er-4a, while the ends of winding 22 are connected to conductor Illa and its associated articial line conductor 19a through condensers 34 and 3| to form opposite, series resonant arm |a-3a. 'I'he arms Iii-3a and '2a-4a; are mutually coupled through the core 23 of coil 2U.

In similar manner coil 20 provides inductance for opposite arms itz-3b and 2li-4b of the Wheatstone bridge connected to .line conductor |011 and its associated artificial line conductor |9b. Winding 2| of coil 28 is connected to line lOb by condenser 2E, and to conductor |919 by condenser 2 to form arm 2li-4b, while winding 22 of coil 20 is connected to line |81) by condenser 32 and to conductor 9b by condenser 33 to form arm lli-3b. The arms IIJ-3b and 2li-4b are mutually coupled through the core 23 of coil 20.

Similarly arm 2c-4c comprises condenser 28, winding 2| and condenser 29 in series while arm |c-3c comprises condenser 34, winding 22 and condenser 35 in series.

It will be evident that the condensers 24, 2E, 28 and 25, 2l, 29 which serve to tune their respective bridge arms comprising winding 2| to resonance at the frequency which is to be suppressed, and condensers 3Q, 32, 34, and 3|, 33, 35 which tune their respective bridge arms comprising winding 22 to resonance, also serve to isolate the three lines from one another Coil 49 has three sets of windings, one for each line conductor. The windings 42, 44 and 46 of coil 4|! are connected to line conductors Illa, Ib and Ic respectively. The windings 43, 45, and 41 are connected to artificial line conductors ld, |95 and |90 respectively All six windings 42 to 4'! inclusive are mounted on a common core 4|, on which is also mounted a winding 48 which is closed through condenser 49. The circuit 48, 49 is so tuned that through the coupling of this tuned circuit to the windings 42, 43, 44, 45, 46, 41, each of these latter windings acts as an antiresonant circuit to current of the frequency to be suppressed.

There are thus provided three pairs of antiresonant arms, one pair for each line conductor and its associated articial line. The opposite arms forming an anti-resonant pair for line ma comprise winding 42 in arm ill-4a and winding 43 in arm Zai-3a; for line i-bg winding 44 in arm |b'-4b and winding 45 in arm 2li-3b; for line |00, winding 4S in arm |c-4c and winding 4l in arm 2c-3c. These anti-resonant arms are all comprised in the single coil 4B and depend for their anti-resonant characteristic upon the common tuned circuit 4S, 49.

Although it might seem that the group of lines would be subject to considerable interference due to coupling, this coupling is in fact negligible except at the resonant frequency. The separate condensers of the diagonal circuits are of the order of 0.1 mf., and since they are connected symmetrically between the lines and artificial lines, only a small amount of far end cross re can result.

It would, of course, be possible, at somewhat greater cost, to provide a separate pair of windings on the diagonal transformer for each wire.

By arranging the cores 23 and 4| of coils 20 and 40 at right angles to one another interaction therebetween is prevented.

What we claim as our invention is:

1. In a telegraph system, the combination of a line conductor subject to disturbing current, a return path for said conductor, a receiver circuit which is to be protected from said disturbing current, a balanced bridge network connected between said line and said return path and to said receiver circuit, said bridge comprising two pairs of opposing arms, the two arms of one of said pairs comprising windings mounted on a common core, and a single means for tuning both said windings to anti-resonance at the disturbing frequency.

2. In a telegraph system, the combination of a line conductor subject to disturbing current, a return path for said conductor, a receiver circuit which is to be protected from said disturbing current, a balanced bridge network connected between said line and said return path and to said receiver circuit, said bridge comprising two pairs of opposing arms, the two arms of one of said pairs comprising windings mounted on a common core and individual condensers for tuning the windings to resonance at said disturbing frequency.

3. In a telegraph system, the combination of a line conductor subject to disturbing current, a return path for said conductor, a receiver circuit which is to be protected from said disturbing current, a balanced bridge network connected between said line and said return path and to said receiver circuit, said bridge comprising a pair of arms series resonant at the disturbing frequency and a second pair of arms comprising two windings of a transformer and means including a third winding of said transformer for tuning said transformer tothe frequency of the disturbing current.

4. In a telegraph system, the combination of a line conductor subject to disturbing current, a return path for said conductor, a receiver circuit which is to be protected from said disturbing current, a balanced bridge network connected between said line and said return path and to said receiver circuit, said bridge comprising two pairs of opposing arms, the arms of one of said pairs comprising two windings of a three-winding transformer and a condenser connected across the third winding to tune the arms of said last mentioned pair to anti-resonance at the frequency of the disturbing current.

5. In a telegraph system, a plurality of line conductors subject td disturbing current, an artificial line balancing each of said conductors, a plurality of balanced bridges one for each line conductor connected between its line conductor and the corresponding artificial line, each bridge comprising four arms resonant at the frequency of the disturbing current, one of said arms comprising an impedance common to all of said balanced bridges.

6. In a telegraph system, a plurality of line conductors' subject to disturbing current, an artificial line balancing each of said conductors, a plurality of balanced bridges one for each line conductor connected between its line conductor and the corresponding articial line, each bridge comprising a pair of opposing arms series resonant at the frequency of said disturbing frequency, the anti-resonant arms of said bridges comprising windings mounted on a common core.

7. In a telegraph system, the combination of a plurality of line conductors subject to disturbing current, return paths for said line conductors, a single suppression network connected to all of said'line conductors and said return paths so as to form a Wheatstone bridge connecting each line conductor and its return path and balanced at the frequency of said disturbing currents, said suppression network comprising irnpedance elements common to all said line conductors.

8. In a telegraph system, the combination of a plurality of line conductors subject to disturbing currents, return paths for said line conductors, a single suppression network connected to all of said line conductors and said return paths comprising a plurality of Wheatstone bridges one for each line conductor, said bridges comprising irnpedance elements common to said line conductors and other impedance elements individual to said line conductors.

9. In a telegraph system, the combination of a plurality of line conductors subject todisturbing currents, return paths for said line conductors, a Wheatstcne bridge connected between each conductor and its return path balanced at the frequency of said disturbing currents, said bridges comprising a coil common to two arms of each bridge and common to all the bridges.

10. In a telegraph system, the combination of a plurality of line conductors subject to disturbing currents, return paths for said line conductors, a Wheatstone bridge connected between each conductor and its return path balanced at the frequency of said disturbing currents, said bridges comprising a coil having windings individual to the line conductors and their return paths but mounted on a common core and another winding mounted on said common core with means for tuning said other winding.

1l. In a telegraph system the combination of a plurality of line conductors subject to disturbing currents, return paths for said line conductors, a Wheatstone bridge connected between each conductor and its return path balanced at the frequency ci said disturbing currents, said bridges comprising a coil common to opposing arms of a pair in each bridge and common to all the bridges.

12. In a telegraph system the combination of a plurality of line conductors subject to disturbing currents, return paths for said line conductors, a Wheatstone bridge connected between each conductor and its return path balanced at the'frequency of Vsaid disturbing currents, said bridges comprising a coil common to all the` nation of a line conductor subject to disturbing current, an artificial line for balancing said line, a receiver circuit which is to be protected from said disturbing current and a bridge network connected between said line and articial line and to said receiver circuit, said bridge comprising four impedance arms connected in lattice formation, two of said arms each including a parallel resonant circuit and the other two arms being constituted by series resonant circuits, the reactances of each of said four arms being resonant at a single frequency and suppressing a narrow range of frequencies while permitting a minimum attenuation at all other frequencies.

15. In a duplex telegraph system, the combination of a line conductor subject to disturbing current, an articial line for balancing said line, a receiver circuit which is t0 be protected from said disturbing current, and a balanced bridge network connected between said line and articial line and to said receiver circuit, said bridge comprising a pair of arms series resonant at the frequency of the disturbing current and a pair of arms anti-resonant at the said frequency, the arms of said rst mentioned pair being mutually coupled.

16. In a duplex telegraph system, the combination of a line subject to disturbing current, an artificial line for balancing said line, and means for suppressing the range of frequencies included in the disturbing current, comprising a 1j bridge network connected between said line and articial line, said bridge having two pairs of opposing arms, each pair being resonant at the frequency of said disturbing current, the impedances of said arms being substantially equal at resonance and approximately equal either to the impedance of the line orto the impedance of the terminal apparatus.

JOSEPH W. MILNOR.

P. F. BECHBERGER.

WILLIAM D. CANNON. 

